1. Field of the Invention
The invention relates to means for use in borehole-seismic measurements at sea, whereby seismic energy sources are placed below the surface of the water suspended on a handling line which is led over a beam arrangement mounted on a vessel.
2. Description of the Prior Art
In recent years borehole-seismic measurements have become an increasingly important method of surveying the conditions around a borehole. This type of measurement, which is also described as VSP ("vertical seismic profiling") is a method of measuring by which a seismic signal generated on the surface is recorded by geophones which are placed at various depths in the borehole. The directional distribution of the geophones used in these measurements will therefore be in vertical direction instead of the side positioning of geophones employed in normal seismic surveys.
The normal procedure in performing borehole seismic measurements at sea has until now been to suspend a seismic energy source on a crane arrangement which can be placed either on the drilling platform itself or, in order to achieve a greater distance and greater variation in relation to the drilling platform, the crane arrangement can be placed on a vessel, which can be positioned in the immediate vicinity of the platform construction. Although it has been possible in this way to achieve a favorable positioning of the actual energy source in relation to the borehole, some difficulties have still arisen when using this measuring technique. When making borehole-seismic measurements it is essential to obtain as clear a signal as possible with regard to frequency and the seismic energy sources employed, which may be waterguns or airguns, are also sensitive with regard to hydrostatic pressure on the energy source, i.e. it is vital to maintain a constant or fixed depth with respect to the surface of the water.
Since it is advisable to work at special frequencies, the size of the energy sources employed is also limited, which means that it may be practical to use more than one source at a time. In order to achieve a pure pulse shape, i.e. without any appreciable fluctuations, it will also be advantageous to work with several seismic sources tuned to each other which can jointly be regarded as a so-called "point formed" transmitter.